1. Field of the Invention
The present invention relates generally to a technique for measuring a distance by using electro-optical means and more specifically to an electro-optical distance meter that enhances accuracy of the measurement and reduces its measuring time.
2. Prior Art
FIG. 18 shows the construction of a conventional and typical Electro-optical Distance Meter (EDM). In this drawing, a light-emitting element 100 emits the light to a triangular prism 104 mounted on a target (not shown) through an object lens 102 to provide an external optical path. The light from the prism 104 is received by a photo-detector element 101 through another object lens 103 to provide its external return path. An internal optical path has a predetermined length and is used for calibrating the external path to be measured. A shutter 105 switching the external and internal optical paths is provided between the photo-elements 100, 101 and object lens 102, 103. The shutter 105 therefore comprises a disc 106 having a pair of openings 107, 107 for measuring the external path and another pair of openings 108, 108 for measuring the internal path. A drive motor 110 rotates the disc 106 intermittently to switch the external path's measurement with the internal path measurement or calibration and vice versa. An electric circuitry 111 includes a circuit for supplying to the light-emitting element 100, an electric signal having a modulated or burst frequency signal, a circuit for converting the optoelectric signal from the photo-detector 101 to a frequency or measuring signal for an operational post-processing, a circuit for comparing the phase difference between the measuring signal and the reference signal to provide another measurement, a circuit for averaging predetermined numbers of said another measurements, and a circuit for compensating the averaged external measurement with the averaged internal measurement.
FIG. 19 is a plan view showing the displacement of the openings each provided on the disc 106 of the optical paths multiplying or switching shutter 105. FIG. 20 is a side view, partially in cross-section, of the major portion of the EDM in which the disc 106 is rotated by 90.degree. from the condition or position of FIG. 18.
In FIG. 19, a pair of the openings 107, 107 is assigned for use in the external measurement. Another pair of the openings 108, 108 is assigned for use in the internal measurement or calibration. Therefore, the disc 105 carries a trapezoidal in cross-section prism 109 having ends in alignment with the openings 108, 108 respectively as shown in FIG. 20.
Upon the external measurement, the openings 107, 107 are pivoted oppose to the light-emitting element 100 and photo-detector 101, respectively, so that the opening 107 transmits the light from the element 100 through the object lens 102 to the target prism 104 in the atmosphere, and that the return light after being reflected by the prism 104 is received by the photo-detector 101 through the object lens 103 and the opening 107. Upon the internal measurement, the openings 108, 108 are pivoted oppose to the light-emitting element 100 and photo-detector 101, respectively, so that the opening 108 transmits the light from the element 100 to one end of the trapezoidal prism 109, and that the return light after being reflected by the prism 109 is received by the photo-detector 101 through the opening 108.
FIG. 21 shows a concrete example of the measuring sequence in the conventional EDM. At first step 201, the shutter 105 is positioned as shown in FIG. 18 to emit the 15 MHz modulating light within 666 milliseconds to the target to provide a plurality of the external measurements. In next step 202, the drive motor 110 is energized to switch the shutter 105 to a second position shown in FIG. 20. It takes about a half second to switch the shutter. In step 203, the 15 MHz modulating light is passed through the trapezoidal prism 109 within 666 milliseconds under the second position to provide a plurality of the internal measurements. In step 204, a frequency selector in the electric circuitry 111 switches its modulating frequency signal to a 150 KHz modulating signal. It takes about 0.2 second to switch the frequency. In step 205, a plurality of second internal measurements are performed in same manner as in step 203 under the second position. In step 206, the shutter 105 is rotated angularly by spending about a half second. Finally, in step 207, a plurality of the 150 KHz external measurements are performed in a similar manner. Then, the 15 MHz or 150 KHz external or internal measurements are averaged to provide an averaged 15 MHz or 150 KHz external or internal measurement, respectively. The 15 MHz or 150 KHz averaged external value is subtracted by the corresponding averaged internal value to provide a 15 MHz or 150 KHz net measurement.
The 15 MHz and 150 KHz modulating frequencies are employed because a coarse measurement is provided with the 150 KHz frequency having a long wavelength or scale, while a fine measurement is provided with the 15 MHz frequency having a short wavelength or scale. As the each measuring time in the respective measurements is set to be 666 milliseconds, 10,000 times sampling measurements are provided. Therefore, the total measuring time in the conventional method is about 3.86 seconds among which about one second is exhausted with the twice shutter switching.
As described above, in the conventional EDM, the measurement modes are performed in a batch fashion, and the disc 106 of the shutter 105 switching the optical paths is angular driven to switch the measurement modes. Then, the measurements are interrupted in long time for switching the modes. The total measurement time is also increased. Therefore, there are some problems in which the EDM can not trace the target to be moved, and its accuracy and reliability are reduced.